This applications claims the benefit of Korean Application No. 00-15572, filed Mar. 27, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device capable of exhibiting dichromatic luminescence while improving luminous efficiency by forming a charge transport buffer layer between light emitting layers.
2. Description of the Related Art
An organic electroluminescent device using an organic compound as the material for forming a light emitting layer provides excellent luminance, driving voltage, and response speed characteristics compared to an inorganic electroluminescent device using an inorganic compound as the material for forming a light emitting layer. Moreover, the organic electroluminescent device described above provides multicolor display.
FIG. 1 shows the basic construction of an organic electroluminescent device 10 described above.
Referring now to FIG. 1, an anode 12 is formed on a substrate 11. A hole transport layer 13, a light emitting layer 14, an electron transport layer 15, and a cathode 16 are sequentially laminated on the anode 12.
The driving principle of the afore-constructed organic electroluminescent device 10 will now be described.
If a voltage is applied between the anode 12 and the cathode 16, holes injected from the anode 12 move to the light emitting layer 14 via the hole transport layer 13. Electrons from the cathode 16 are injected into the light emitting layer 14 via the electron transport layer 15, and carriers (that is, the electrons and the holes) are recombined at the interface between the hole transport layer 13 and the light emitting layer 14 (the electron transport layer 15 itself may function as a light emitting layer) to generate excitons. These exciton are turned from an excitation state into a ground state, thus releasing energy and emitting light. Accordingly, fluorescent molecules of the light emitting layer 14 radiate light to form an image.
The organic electroluminescent device operating based on the above-described driving principle improves the luminous efficiency by enhancing charge balance. However, a carrier recombining region is formed at the interface between a hole transport layer and a light emitting layer (or electron transport layer) to form an exciplex, thereby lowering the luminous efficiency. Also, it is difficult to apply the organic electroluminescent device to exhibit dichromatic luminescence such as white luminescence.
To solve the above-described problem, U.S. Pat. No. 5,283,132 discloses an organic electroluminescent device exhibiting white luminescence. In the organic electroluminescent device disclosed in U.S. Pat. No. 5,283,132, a hole transport layer is formed from bis-di(p-tolyl)aminophenyl-1,1-cyclohexane, a luminescent layer is formed from a material having a luminescent peak at 460-480 nm, and the thickness of the luminescent layer is adjusted to be in the range of 100-300 xc3x85, thereby obtaining an organic electroluminescent device capable of exhibiting white luminescence.
However, in practice in the organic electroluminescent device described above, it is difficult to adjust white balance properly due to a change in the blue color coordinate characteristic.
To solve the above problems, it is an object of the present invention to provide an organic electroluminescent device which prevents lowering of luminous efficiency due to formation of an exciplex by forming a charge transport buffer layer to cause excitons to be produced directly on a separate light emitting region rather than the interface between a hole transport layer and a light emitting layer and which achieves materialization of dichromatic luminescence by adjusting the thickness of the charge transport buffer layer.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above object, the present invention comprises an organic electroluminescent device comprising a substrate, and an anode, a hole transport layer, a plurality of light emitting layers, an electron transport layer and a cathode in laminated sequentially on the substrate. In the organic electroluminescent device of the present invention, a charge transport buffer layer adjusting the proportion generating excitons of two light emitting layers by adjusting a film thickness is formed between the light emitting layers.